Local cardiac motion control using applied signals and mechanical force

ABSTRACT

Apparatus ( 18 ) for performing a medical procedure on a beating heart ( 20 ) includes a mechanical stabilization element ( 25 ), a surface ( 27 ) of which is adapted to be applied to a segment ( 24 ) of the heart to reduce motion of the segment. One or more electrodes ( 100 ) are fixed to the surface of the stabilization element, so as to contact the segment when the stabilization element is applied to the segment. Preferably, at least one of the one or more electrodes is adapted to apply electrical signals to the segment so as to further reduce motion thereof, while the heart continues to pump blood.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of commonly assigned, co-pending U.S.patent application Ser. No. 09/979,937, filed May 20, 2002; which inturn is a U.S. National Phase of PCT Patent Application No.PCT/IL00/00303, filed May 25, 2000, which in turn is based upon U.S.patent application Ser. No. 09/320,090, filed May 26, 1999, now U.S.Pat. No. 6,442,424, each of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to invasive devices and methodsfor treatment of biological tissue, and specifically to devices andmethods for controlling tissue and muscle during surgery.

BACKGROUND OF THE INVENTION

Heart surgery is often accompanied by the induction of cardioplegia(elective stopping of essentially all cardiac activity by injection ofchemicals, selective hypothermia, mechanical stabilization, orelectrical stimuli). In humans, induced global cardioplegia is nearlyalways practiced in conjunction with cardiopulmonary bypass.

Recently, minimally-invasive methods of cardiac surgery have beendeveloped, in which the heart is approached through an incision madebetween the ribs, without stemotomy. It is sometimes preferred that,rather than inducing cardioplegia, the surgeon mechanically restrains aportion of the heart on which a surgical procedure, such as a bypassgraft, is to be performed. Various tools and methods have been developedfor this purpose, such as: (a) a suction cup-based stabilizationplatform (e.g., the Utrecht Octopus); (b) mechanical stabilizationdevices, such as the Ultima OPCAB System, produced by Guidant, Inc.(Indianapolis, Ind.); (c) the Octopus 2 or the EndoOctopus device, bothproduced by Medtronic, Inc. (Minneapolis, Minn.); (d) a U-shaped metalfoot and other stabilizers produced by Genzyme Surgical Products, Inc.(Tucker, Ga.); (e) the Octopus Suction stabilizer, produced by MedtronicGmbH, Germany; and (f) CardioVations mechanical stabilizers produced byEthicon Endo-Surgery (Cincinnati, Ohio).

The ClearView Blower/Mister, produced by Medtronic, is used duringminimally-invasive cardiac surgery to spray a gas/saline mist into theoperative field, so as to remove blood therefrom. The surgeon bends thedevice into a desired shape and places it near to the operative field,so that only the target area will be sprayed. In a product descriptionon the World Wide Web (http: II www.medtronic.com/cardiac/mics/prodclearview.html, Feb. 8, 2000), Medtronic suggests that “generous tubelength provides access to the surgical site, while the user's handremains outside the surgical field.”

An article entitled “Coronary artery bypass grafting withoutcardiopulmonary bypass and without interruption of native coronary flowusing a novel anastomosis site restraining device (‘Octopus’),” by Borstet al., Journal of the American College of Cardiology, 27(6) (May 1996),pp. 1356-1364, which is incorporated herein by reference, describes useof the Octopus suction-generating device during experimental surgery onin situ pig hearts.

Such mechanical restraint of the heart muscle requires that substantialforce, e.g., pressure or vacuum, be applied, which can cause tissuetrauma. The effects of mechanical stabilization are described in anarticle, “The effects of mechanical stabilization on left ventricularperformance,” by Burfeind et al., European Journal of Cardio-ThoracicSurgery, 14(1998), pp. 285-289, which is incorporated herein byreference.

PCT Patent Publication WO 97/25098, to Ben-Haim et al., and thecorresponding U.S. National Phase patent application Ser. No.09/101,723, entitled, “Electrical muscle controller,” which are assignedto the assignee of the present patent application and are incorporatedherein by reference, describe methods for modifying the force ofcontraction of at least a portion of a heart chamber by applying anon-excitatory electrical signal to the heart at a delay afterelectrical activation of the portion. The signal may be applied incombination with a pacemaker or defibrillator, which also applies anexcitatory signal (i.e., pacing or defibrillation pulses) to the heartmuscle.

PCT Patent Publication WO 98/10832 to Ben-Haim et al., and thecorresponding U.S. National Phase patent application Ser. No.09/254,900, entitled, “Cardiac output enhanced pacemaker,” which arealso assigned. to the assignee of the present patent application andincorporated herein by reference, describe a pacemaker that modifiescardiac output. This pacemaker applies both excitatory (pacing) andnon-excitatory electrical signals to the heart. By applyingnon-excitatory signals of suitable strength, appropriately timed withrespect to the heart's electrical activation, the contraction ofselected segments of the heart muscle can be increased or decreased.U.S. Pat. No. 5,651,378, to Matheny et al., and an article entitled,“Vagus Nerve Stimulation as a Method to Temporarily Slow or Arrest theHeart,” by Matheny and Shaar, Annals of Thoracic Surgery, 63(6)Supplement (June 1997), pp. S28-29, which are both incorporated hereinby reference, describe a method to stimulate the vagus nerve in order toslow or stop a patient's heart during coronary artery bypass graftingsurgery. In addition, an article entitled “Right vagal nerve stimulationduring minimally invasive direct coronary artery bypass grafting indogs: A preliminary study,” by Hayashi et al, Journal of CardiovascularSurgery (Torino), 39(4) August 1998, pp. 469-471, which is incorporatedherein by reference, describes a first set of experiments, in which thevagal nerve was stimulated so as to slow the heart rate. In a second setof experiments, the calcium channel blocking agents diltiazem orverapamil were administered in conjunction with the vagal nervestimulation, and produced either marked bradycardia or ventriculararrest. It is noted that stimulation of the vagus nerve affects not onlycardiac function, but also the functioning of other parts of thepatient's body, such as the pharynx, larynx, trachea, lungs, andgastrointestinal tract.

SUMMARY OF THE INVENTION

It is an object of some aspects of the present invention to provideimproved methods and apparatus for regulating the heart.

It is a further object of some aspects of the present invention toprovide improved methods and apparatus for reducing motion of the heartduring minimally-invasive and open chest surgery.

It is yet a further object of some aspects of the present invention toprovide improved methods and apparatus for applying mechanical force toreduce motion of the heart during minimally invasive and open chestsurgery.

It is still a further object of some aspects of the present invention toprovide improved methods and apparatus for reducing motion of the heartduring minimally-invasive and open-chest surgery, while minimizing orsubstantially eliminating injury to the heart resulting from the motionreduction.

In preferred embodiments of the present invention, tissue controlapparatus inhibits motion of a segment of a patient's heart, whileallowing the heart to continue to pump blood. The tissue controlapparatus comprises a stabilization element, which has a surface that isapplied to the heart in order to reduce motion thereof. Additionally,one or more electrodes are coupled to the surface of the stabilizationelement. When the element is applied to the segment of the heart, acontrol unit applies electrical signals to the heart through at leastone of the electrodes, so as to reduce or substantially stop motion ofthe segment for the duration of signal application. Alternatively oradditionally, the signals are applied through the element so as tocontrol other aspects of the mechanical behavior of the patient's heart.Further alternatively or additionally, the control unit detectselectrical activity of the segment of the heart through the electrodescoupled to the stabilization element. At generally the same time, thestabilization element applies a mechanical motion-restraining force tothe segment of the heart, so as to further reduce the segment's motion.Termination of signal and force application allows the segment, as wellas the heart as a whole, to resume substantially normal motion.

The reduction in motion of the segment, as provided by these embodimentsof the present invention, is typically used to enable a surgeon toperform minimally invasive surgery or open-chest surgery, generallywithout inducing global cardioplegia or requiring cardiopulmonarybypass. For some applications, the electrical signals are used to reducethe force applied—and thus the injury produced—by the stabilizationelement, while maintaining a desired level of motion reduction. Purelymechanical stabilization devices known in the art, by contrast, reducemotion of a segment of the heart through application of a mechanicalforce to the delicate tissue of the heart that is considerably largerthan that generated using these embodiments of the invention, andtherefore risk damaging the tissue which is being forced to besubstantially motionless.

In some preferred embodiments of the present invention, one or moremotion sensors, e.g., accelerometers, are coupled to the stabilizationelement and/or to the heart, and send motion signals to the control unitindicative of the segment's motion and, optionally, of the motion ofother areas of the heart. Preferably, the motion signals serve asfeedback to enable the control unit to adjust the electrical signalsapplied to the heart, in order to reduce or otherwise regulate thedetected motion of the segment. In a preferred embodiment, the controlunit receives the motion signals from the sensors, and actuates theelectrodes to apply the electrical signals in order to changecontractility and/or contraction timing of muscle in the segment, so asto reduce the detected motion.

The electrical signals applied to the heart preferably comprise one ormore of: regular pacing pulses, rapid pulses, a fencing signal (asdescribed hereinbelow), and an enhancement signal. The enhancementsignal is typically similar to an Excitable-Tissue Control (ETC) signal,as described in the above-referenced PCT Patent Publication WO 97/25098,U.S. patent application Ser. No. 09/101,723, and in U.S. patentapplication Ser. No. 09/260,769, entitled “Contractility enhancementusing excitable tissue control and multi-site pacing,” which is assignedto the assignee of the present patent application and incorporatedherein by reference. While for some applications these signals areapplied so as to reduce motion of the heart, they may alternatively oradditionally be applied to modify the mechanical behavior of the heartin other ways, such as those described in one or more of the patentapplications incorporated herein by reference. Most preferably, theelectrical signals are synchronized with the overall heartbeat, and havetiming, shape, and magnitude characteristics which are determined duringa calibration period of the control unit. As a result of calibration ofthe tissue control apparatus, a high degree of stabilization ispreferably achieved, while maintaining adequate safety margins, e.g.,acceptable patient vital signs, reduction in applied mechanical force,and avoidance of fibrillation and arrhythmia.

Generally, motion of the segment is characterized by a sum of (a) afirst component, consisting of motion resulting from general contractionand relaxation of the heart; and (b) a second component, consisting oflocal motion due to stimulation of the segment by the electrodes on thestabilization element, and due to the motion-restraining force generatedby the stabilization element. It is a goal of this embodiment of thepresent invention to apply electrical signals which alter the secondcomponent, particularly with respect to the timing thereof, such thatthe net motion of the segment, resulting from summing the twocomponents, is generally minimized and/or smoothed.

In some preferred embodiments of the present invention, additionalelectrodes are placed at multiple sites on the epicardium and/orendocardium of the segment of the heart. Alternatively or additionally,the additional electrodes are placed in blood vessels of the heart or ina vicinity of the heart, and, optionally, on areas of the heart otherthan the segment. Typically, each of the additional electrodes conveys aparticular waveform to the heart, which may differ in certain aspectsfrom the waveforms applied to other electrodes. The particular waveformto be applied to each electrode is preferably determined by the unitunder the control or supervision of a human operator, in such a manneras to regulate the first and/or the second component of the segment'smotion.

U.S. patent application Ser. No. 09/320,091, entitled, “Induction ofcardioplegia using applied electrical signals,” which is assigned to theassignee of the present invention and is incorporated herein byreference, describes methods for applying electrical signals to theheart to induce a global cardioplegic state. Additionally, theabove-mentioned U.S. patent application Ser. No. 09/320,090 describesmethods and apparatus for reducing the motion of a segment of the heart.Aspects of the methods described in these patent applications may alsobe used in conjunction with the principles of the present patentapplication. In particular, in a preferred embodiment of the presentinvention, the electrical signals applied to the heart comprise rapidpulses and/or fencing signals, as described hereinbelow, applied throughone or more of the electrodes coupled to the stabilization element, inorder to induce a state of generally constant and/or reduced contractionof the segment. The use of such pulses is described further inapplication Ser. Nos. 09/320,091 and 09/320,090. Additionally, thesignals may be applied to other regions of the heart in order to modifycontraction parameters in the other regions (e.g., timing and strength),such that motion of the segment is reduced. Alternatively oradditionally, rapid pulses and/or other signals are applied usingmethods and apparatus described in a US patent application filed May 5,2000, entitled “High-frequency induction of cardioplegia,” which isassigned to the assignee of the present patent application and isincorporated herein by reference.

In some preferred embodiments of the present invention, a “fencing”signal is applied through one or more of the electrodes, preferably inorder to prevent or inhibit the propagation of an action potential fromone region of the heart to another. Fencing may be applied inconjunction with any (or none) of the electrical signals describedhereinabove. Most preferably, the fencing signal is applied in avicinity of the segment. Such fencing is described in PCT PatentPublication WO 98/10830 and U.S. patent application Ser. No. 09/254,903,both of which are entitled, “Fencing of cardiac muscles,” assigned tothe assignee of the present invention, and incorporated herein byreference. Fencing is typically used, according to these embodiments, toreduce motion and/or a contraction force of the segment, generally byblocking or reducing the normal propagation of signals, and sometimes byapplying the fencing signal to one or more sites within the segment.

In some preferred embodiments of the present invention, periods ofmechanical force and electrical signal application are separated byperiods in which force and/or electrical signals are not applied.Preferably, the durations of the application and non-application periodsare set to maximize the surgeon's time for performing surgery, withoutunnecessarily extending the length of time in which free motion of thesegment is limited. It is noted, however, that even in applicationswhich utilize continuous application to the segment of a stabilizingforce and/or motion-reduction signals, the level of functioning of therest of the heart is expected to be generally sufficient to supportsystemic activity without the need for cardiopulmonary bypass.

For some applications, it may be desirable to partially (and, in somecases, significantly) reduce the overall output of the heart in order toattain a high degree of stabilization of the segment for a short time.Suitable methods of electrical control of the heart to reduce cardiacoutput are described in the above-mentioned U.S. patent application Ser.Nos. 09/101,723 and 09/254,900 and in PCT Patent Publications WO97/25098 and WO 98/10832. It is emphasized that in these embodiments, asin most applications of the present invention, the patient's vital signsare preferably monitored substantially continuously.

In some preferred embodiments of the present invention, an automatic orsemi-automatic feedback loop modifies the electrical signals applied tothe heart, so as to optimize the segment's stabilization withoutundesirably changing measured physiological parameters, such as, forexample, pCO2, pO2, Left Ventricular Pressure (LVP), ECG, and systemicblood pressure. Preferably, an abnormal value of any of these parameterstriggers an alarm, responsive to which the operator and/or the controlunit initiates an appropriate response. Further preferably, arrhythmiaand fibrillation detection capabilities, as well as appropriatetreatment protocols, are incorporated into the control unit.

Preferably, application of the mechanical force and electrical signalsin accordance with the present invention stabilizes the segment within avery short period, typically about 1 second, and can maintain thesegment's stability for prolonged periods. The heart typically returnsto normal function within about 2 seconds of removal of the electricalsignals.

There is therefore provided, in accordance with a preferred embodimentof the present invention, apparatus for performing a medical procedureon a beating heart, including:

-   -   a mechanical stabilization element, a surface of which is        adapted to be applied to a segment of the heart to reduce motion        of the segment; and    -   one or more electrodes, fixed to the surface of the        stabilization element, so as to contact the segment when the        stabilization element is applied to the segment.

Preferably, at least one of the one or more electrodes is adapted toapply electrical signals to the segment so as to further reduce motionthereof, while the heart continues to pump blood.

In a preferred embodiment, the one or more electrodes include one ormore local sense electrodes, and the apparatus includes a control unit,coupled to the local sense electrodes. Preferably, the local senseelectrodes are adapted to convey to the control unit a currentresponsive to electrical activity of the heart, and the control unit isadapted to modify the electrical signals responsive to the conveyedcurrent.

Alternatively or additionally, the at least one of the one or moreelectrodes is adapted to apply the signals so as to modify contractionof muscle tissue of the heart.

Preferably, the at least one of the one or more electrodes is adapted toapply the signals at a rate greater than about 5 Hz.

In a preferred embodiment, the at least one of the one or moreelectrodes includes two electrodes, which are adapted to concurrentlyapply to the segment respective first and second electric fields atrespective first and second frequencies, so as to generate a field inthe heart at a beat frequency of the first and second frequencies whichreduces motion of the segment.

Alternatively or additionally, the at least one of the one or moreelectrodes is adapted to apply to the segment an electric field having acarrier frequency in excess of about 500 Hz, an amplitude of whichelectric field is modulated at a modulation frequency, so as to reducemotion of the segment.

In a preferred embodiment, the one or more electrodes include one ormore fencing electrodes, which are adapted to apply a fencing signal tothe heart so as to block propagation of an activation wave into thesegment. Alternatively or additionally, the one or more electrodesinclude one or more fencing electrodes, which are adapted to apply afencing signal to the heart so as to reduce a contraction force thereof.

Optionally, the one or more electrodes include one or more pacingelectrodes, which are adapted to apply a pacing signal to the heart.Further optionally, the one or more electrodes include one or moreenhancement electrodes, which are adapted to apply an enhancement signalto the heart. Still further optionally, the one or more electrodesinclude one or more local sense electrodes, which are adapted to senseelectrical activity of the heart.

Typically, the one or more electrodes include at least one carbonelectrode, stitch electrode, wire electrode, and/or needle electrode.

In a preferred embodiment, the apparatus includes a transport element,fixed to the stabilization element, which transport element is adaptedto convey a fluid between the segment of the heart and the stabilizationelement, when the stabilization element is applied to the segment.

There is further provided, in accordance with a preferred embodiment ofthe present invention, a method for performing a medical procedure on abeating heart, including:

-   -   applying a mechanical stabilization element to a segment of the        heart, so as to reduce motion of the segment; and    -   conveying electrical signals between the segment and the        element.

Typically, performing the procedure includes performing a treatment onthe segment while motion of the segment is reduced. Alternatively oradditionally, performing the procedure includes performing a diagnosticprocedure while motion of the segment is reduced.

Preferably, applying the signals includes applying bipolar and/orunipolar signals, as well as calibrating the signals intermittentlyduring the procedure.

In a preferred embodiment, applying the signals includes:

-   -   applying a first signal, prior to performing the procedure, so        as to precondition a response of the heart; and    -   applying a subsequent signal, during the procedure, so as to        reduce the motion of the segment during the procedure.

Alternatively or additionally, applying the signals includes:

-   -   sensing electrical activity of the heart to detect arrhythmia        thereof; and    -   applying electrical energy to the heart to treat the arrhythmia.

Further alternatively or additionally, applying the signals includes:sensing electrical activity of the heart; and

-   -   modifying the application of the electrical signals responsive        to the sensed electrical activity.

Preferably, the method includes sensing motion of the heart, whereinapplying the signals includes modifying a characteristic of at leastsome of the signals applied to the heart responsive to the sensedmotion.

In a preferred embodiment, applying the signals includes applying afencing signal to the heart to block propagation of an activation waveinto the segment of the heart and/or to reduce a contraction forcethereof.

Typically, applying the signals includes applying the signals so as tofurther reduce motion of the segment.

In a preferred embodiment, applying the signals includes applying pulsesat a rate greater than 5 Hz.

In a preferred embodiment, applying the electrical signals includesapplying to the segment first and second electric fields at respectivefirst and second frequencies, so as to generate a field in the heart ata beat frequency of the first and second frequencies which reducesmotion of the segment. Alternatively or additionally, applying theelectrical signals includes applying to the segment an electric fieldhaving a carrier frequency in excess of about 500 Hz, an amplitude ofwhich electric field is modulated at a modulation frequency, so as toreduce motion of the segment.

Typically, applying the signals includes applying pulses and/or anenhancement signal to the segment.

In a preferred embodiment, applying the signals includes applying thesignals so as to modify contraction of muscle tissue of the heart.Typically, modifying the contraction includes inducing contraction ofthe muscle tissue. Alternatively or additionally, applying the signalsincludes:

-   -   determining an aspect of the motion of the segment due generally        to contraction of muscle tissue outside the segment; and    -   adjusting the signals responsive to the determined aspect of the        segment's motion, so as to reduce the aspect of the segment's        motion.

In a preferred embodiment, applying the signals includes applyingsignals through the stabilization element to a plurality of sites on thesegment of the heart. For example, applying the signals may includeapplying a first waveform at a first one of the sites and applying asecond waveform, which differs from the first waveform, at a second oneof the sites. Preferably, applying the first and second waveformsincludes controlling a timing relationship of the waveforms so as toreduce the motion of the segment.

There is still further provided, in accordance with a preferredembodiment of the present invention, apparatus for performing a medicalprocedure on a beating heart, including:

-   -   a stabilization element, a surface of which is adapted to be        applied to a segment of the heart to reduce motion of the        segment; and    -   one or more transport elements, fixed to the stabilization        element, which are adapted to convey a fluid between the segment        of the heart and the stabilization element, when the        stabilization element is applied to the segment.

Preferably, one of the one or more transport elements is adapted toapply a liquid and/or gas to the segment of the heart.

Alternatively or additionally, one of the one or more transport elementsis adapted to apply suction, so as to remove liquid from the surface ofthe heart.

There is yet further provided, in accordance with a preferred embodimentof the present invention, a method for performing a medical procedure ona beating heart, including:

-   -   applying a stabilization element to a segment of the heart, so        as to reduce motion of the segment; and    -   conveying a fluid between the segment and the element, when the        stabilization element is applied to the segment.

The present invention will be more fully understood from the followingdetailed description of the preferred embodiments thereof, takentogether with the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the external surface of a heart,showing the placement of a stabilization element thereon, in accordancewith a preferred embodiment of the present invention;

FIG. 2A is a schematic illustration of the stabilization element, inaccordance with a preferred embodiment of the present invention;

FIG. 2B is a schematic illustration of the stabilization element, inaccordance with another preferred embodiment of the present invention;

FIG. 2C is a schematic illustration of the stabilization element, inaccordance with yet another preferred embodiment of the presentinvention; and

FIG. 3 is a schematic block diagram of a control unit, which generatessignals to be applied to the heart through the stabilization element, inaccordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is made to FIGS. 1, 2A, 2B, and 2C. FIG. 1 is a schematicillustration of apparatus 18, comprising a stabilization element 25 forreducing the motion of a segment 24 of a patient's heart 20, inaccordance with a preferred embodiment of the present invention. FIGS.2A, 2B, and 2C are schematic illustrations of stabilization element 25,in respective configurations thereof, in accordance with preferredembodiments of the present invention. Two techniques are typicallyutilized concurrently to modulate the motion of segment 24, in order toenable surgery within the segment:

Mechanical stabilization: A surface 27 of stabilization element 25 isplaced on segment 24, so as to apply a mechanical force thereto. Theforce is typically derived from a positive pressure exerted by surface27 of the element on heart 20. Alternatively or additionally, one ormore optional vacuum ports 39 on surface 27 are coupled through acontrol unit 90 of apparatus 18 to a liquid-gas-vacuum source 92. Thevacuum ports hold the surface of the heart in contact with thestabilization element, thereby reducing motion of segment 24. Typically,but not necessarily, surface 27 is roughened or otherwise configured soas to reduce or eliminate any slip between surface 27 and segment 24.

(2) Electrical Stimulation and/or sensing: One or more electrodes 100coupled to surface 27 are actuated by control unit 90 to applyelectrical signals to heart 20 and/or to sense electrical activity ofthe heart. Preferred parameters of the signals are described hereinbelowwith reference to FIG. 3. The electrical signals typically reduce motionof the heart, but may, alternatively or additionally, pace the heart orintermittently enhance or otherwise modulate motion of the heart.

Typically, application of signals as provided by these embodiments ofthe present invention enables the mechanical force applied by element 25to be reduced compared to forces applied using strictly mechanicalcardiac stabilizers known in the art. Moreover, the reduced mechanicalforce is generally achieved while maintaining at least the same level ofmotion reduction as is yielded using the prior art stabilizers. Theinventors believe that reducing the applied force, as is enabled usingthese embodiments of the present invention, minimizes injury to tissueof the heart that may be produced using the prior art mechanicalstabilizers. Additionally, use of mechanical stabilization inconjunction with the electrical signals may reduce motion of the segmentto a level below that which could safely be attained by applyingmechanical force or electrical signals separately.

Optionally, a handle 41 of stabilization element 25 has two members 29and 31, which are slidably coupled to each other. Lines 35 preferablypass through handle 41, to couple control unit 90 to electrodes 100 andvacuum ports 39. For some applications, lines 35 couple additionalelectrodes, sensors, and actuated devices on the stabilization elementto control unit 90. To simplify the performance of open-chestprocedures, a connecting member 37 typically couples handle 41 to achest retractor (not shown), so as to keep the stabilization elementgenerally stationary with respect to the patient's chest.

An elevated portion 33 of stabilization element 25 preferably enablessurface 27 of the element to be placed on segment 24, without directlycompressing a particular site within the segment. Thus, for example,elevated portion 33 may be placed over the left anterior descendingartery 22 of heart 20, so as not to restrict blood flow through theartery. Preferably, one or more liquid/gas transport elements 62 onelevated portion 33 or elsewhere on the stabilization element apply oneor more materials, such as physiological saline solution, gaseous CO₂,and/or air to the surface of heart 20, so as to keep the surgery sitemoist and/or clear of blood. Further preferably, the flow of thesematerials through transport elements 62 is regulated by control unit 90,which is coupled to control flow from liquid-gas-vacuum source 92.Alternatively or additionally, elements 62 apply suction to the surgerysite, so as to remove therefrom blood or other liquids that mayinterfere with the surgeon's work.

In a preferred embodiment of the present invention, transport elements62 are coupled to stabilization element 25 in the absence of electrodescoupled thereto. Prior art transport elements, such as the ClearViewBlower/Mister described in the Background section of the presentapplication, are entities unto themselves, which must be deliberatelyplaced in the operative field, and subsequently maintained there, oftenby a person other than the surgeon. In this embodiment of the presentinvention, by contrast, transport elements 62 are coupled to thestabilization element, which will in any case be placed and maintainedin the operative field (typically using connecting member 37).

Depending on the patient's condition and the site of the surgery, asurgeon will typically select a stabilization element in whichelectrodes 100 comprise one or more of the following types ofelectrodes: carbon electrodes 34 (FIG. 2A), needle electrodes 52 (FIG.2B), or wire electrodes 54 (FIG. 2C). As appropriate, other types ofelectrodes may be incorporated into stabilization element 25, inaddition to or instead of those shown. Alternatively or additionally,more or fewer electrodes may be incorporated into the stabilizationelement. In the preferred embodiment shown in FIG. 2A, for example,dedicated local sense electrodes 74 coupled to stabilization element 25convey electrical signals to control unit 90 responsive to cardiacelectric activity. Alternatively or additionally, some or all ofelectrodes 100 (FIGS. 2B and 2C) convey signals to the control unitresponsive to the heart's electrical activity, without the use ofdedicated local sense electrodes.

The types and placement of electrodes and sensors in FIGS. 2A, 2B, and2C are shown by way of example. Other sites on stabilization element 25,or in and around the heart, are appropriate for electrode or sensorplacement in other applications of the present invention. In particular,electrodes may be placed in a manner similar to that described in theabove-mentioned U.S. patent application Ser. No. 09/320,090, entitled“Local cardiac motion control using applied electrical signals.”Additionally, different numbers of electrodes or sensors (including noelectrodes or sensors in some areas) and different types andcombinations of sensors and coil, stitch, defibrillation, basket, screw,patch, needle and wire electrodes may be used in applying the principlesof the present invention. It is noted that whereas specific types andplacements of electrodes are described herein and shown in the figures,it is within the scope of the present invention to use, as appropriate,substantially any electrodes known in the art of tissue stimulation andbioelectrical sensing, and to place these electrodes on stabilizationelement 25, at one or more locations on or in a vicinity of the heart,or elsewhere on or in the patient's body.

In addition to the electrodes described hereinabove, a plurality ofmotion sensors 70 (e.g., accelerometers) and one or more optionalsupplemental sensors 72 are preferably coupled to stabilization element25 (FIG. 2B), to the heart, or to another site on or in the patient'sbody. Sensors 72 may comprise, for example, a systemic blood pressuresensor, an LVP sensor, a p02 sensor, a pC02 sensor, a flow rate sensor,and/or a force sensor, which measures a contact force betweenstabilization element 25 and heart 20. The electrodes and sensors(optionally in combination with other electrodes and sensors not coupledto the stabilization element) provide substantially continuousmonitoring of the patient's vital signs, in order to ensure that all ofthe signs are maintained within a safe range during the surgery. To theextent that any of the vital signs is outside the range, control unit 90will either take corrective action on its own and/or provide an alarm tothe surgeon, who will then be able to respond appropriately.

FIG. 3 is a schematic block diagram of control unit 90, which conveyselectrical energy to stabilization element 25 in order to reduce motionof segment 24, in accordance with a preferred embodiment of the presentinvention. Alternatively or additionally, the control unit conveys tothe stabilization element other forms of electrical energy, such asstandard pacing pulses or the enhancement signal described in theSummary section of this application. Preferably, control unit 90 conveysthe electrical energy to one or more of electrodes 100 coupled tosurface 27 of stabilization element 25, in order to reduce orsubstantially stop the motion of segment 24. In a preferred embodiment,local sense electrodes 74 and/or electrodes 100 convey signalsresponsive to the electrical activity of heart 20 to the control unit,and substantially no signals are applied to the heart through thestabilization element.

Motion sensors 70, described hereinabove with reference to FIG. 2B,preferably send motion sensor signals to a motion analysis block 80 ofcontrol unit 90. The motion sensor signals provide feedback to thecontrol unit, which modifies the electrical signals applied to the heartresponsive thereto. For example, the electrical signals may includepulses, characteristics of which are adjusted by the control unitresponsive to the motion sensor signals, in order to minimize motion ofsegment 24. Motion analysis block 80 5 preferably comprises amplifiersto amplify low-level signals generated by motion sensors 70, and asignal processing block, coupled to the amplifiers, which determinesrespective states of motion of the motion sensors. In some applications,motion analysis block 80 additionally receives signals from one or moreof supplemental sensors 72, particularly those sensors that detectmechanical phenomena such as blood flow rate and blood pressure.

Preferably, motion analysis block 80 conveys results of its analysis toa “parameter search and tuning” block 84 of control unit 90, whichiteratively modifies characteristics of the electrical signals in orderto reduce the motion of segment 24. To achieve this goal, block 84typically utilizes multivariate optimization and control methods knownin the art (e.g., downhill simplex, linear state variable feedback orextended Kalman filters), in order to cause the measured motion and/orother parameters to converge to a desired value. For the purposes ofsome embodiments of the present invention, block 84 modifies a set ofcontrollable parameters to minimize and/or smooth motion of segment 24.Preferably, the controllable parameters are conveyed by block 84 to asignal generation block 86 of control unit 90, which generates,responsive to the parameters, electrical signals that are applied byelectrodes 100 to segment 24.

As described hereinabove, motion sensors 70 are generally attached tostabilization element 25 and/or directly to the heart. Typically, themotion sensors are mechanically coupled to segment 24, and the elementis placed adjacent to a surgical location within the segment. In theembodiment shown in FIG. 1, for example, the stabilization element isplaced on the surface of left ventricle 44, adjacent to the leftanterior descending artery 22, to enable a single-vessel coronary arterybypass graft to be performed thereon. Alternatively, the stabilizationelement is placed at another ventricular site, or, for someapplications, at an atnal site of heart 20. Typically, the control unitreceives motion signals from sensors 70 indicative of motion of thesurgical site, and actuates electrodes 100 to apply the electricalsignals in order to cause muscle in a vicinity of the site to contractin a manner which generally reduces motion of the site.

Generally, motion of segment 24 is characterized by a sum of: (a) afirst component, consisting of global heart motion resulting frombeating of heart 20, and especially motion due to contraction of heartregions not within segment 24; and (b) a second component, consisting ofmotion resulting from the part of the heart in segment 24 that istypically stimulated by electrodes 100. Control unit 90 generallyapplies the electrical energy to electrodes 100 on the stabilizationelement so as to alter the second component of the motion. In apreferred embodiment, additional electrodes (not shown) are applieddirectly to the heart, independent of stabilization element 25. Theseadditional electrodes apply other signals to the rest of the heart, soas to alter the first component (and, particularly, to alter timing ofthe first component), such that the net motion of segment 24, resultingfrom summing the two components, is generally minimized and/or smoothed.Electrodes suitable for direct placement on the heart are described inthe above-mentioned U.S. patent application Ser. No. 09/320,090,entitled “Local cardiac motion control using applied electricalsignals.”

Preferably, the electrical signals provided by some embodiments of thepresent invention have some similarity to pacing pulses, and/or aretimed to correlate with pacing pulses. They are typically synchronizedwith the overall heartbeat, and have timing, shape, and magnitudecharacteristics which are determined during a calibration period at thebeginning of a surgical procedure and/or at regular intervals during theprocedure. For some applications, the electrical signals applied to theheart comprise combinations of signals described herein, includingregular pacing, rapid pulses, fencing, enhancement signals and othersignals.

During the calibration period, parameter search and tuning block 84preferably executes an optimization algorithm, such as “gradientdescent,” in which, for example, block 84 modifies a characteristic(e.g., timing, duration, or magnitude) of the electrical signalsgenerated by one of the electrodes described herein, and then determineswhether the measured motion of segment 24 decreases, or changes in someother desired way, following the modification. Typically, in a series ofsimilar calibration steps, block 84 modifies characteristics of theelectrical signals applied by each of the other electrodes, whereinthose modifications that reduce motion of segment 24 are generallymaintained, and modifications that increase the motion of the segmentare eliminated or avoided. In combination with the application oflimited mechanical force by stabilization element 25, motion of segment24 is gradually reduced to a point at which the surgeon can safely andconveniently perform the surgical procedure. Optionally, the surgeondoes not use the stabilization element to apply mechanical force untilmotion of segment 24 has already been substantially reduced through theapplication of the electrical signals.

In some cases, it is desirable to have a preconditioning period ofsegment 24 and/or of the whole heart. During the preconditioning period,electrodes 100 (or other electrodes placed on the heart) apply theelectrical signals for short periods initially, followed byprogressively longer periods. During the preconditioning period,characteristics of the heart's response to the signals change, so thatsubstantially similar inputs will engender different responses beforeand after the preconditioning period. In a preferred embodiment, thecontrol unit applies signals for a 2 second period, followed by 4second, 6 second, and longer periods, until a desired motion-reductionperiod of 20 seconds is attained. It is believed that the heart ispreconditioned, or trained, during this period, and that training theheart during the preconditioning period may improve the response of theheart during subsequent signal-application periods. Because the heartmay change its response to the applied signals throughout the surgicalprocedure, i.e., it is continually being trained, it is generallypreferable to repeat the calibration at intermittent times during theprocedure.

Most preferably, during the calibration period and during regularoperation of control unit 90, an arrhythmia detection block 82 ofcontrol unit 90 receives inputs from motion sensors 70, supplementalsensors 72, electrodes 74 and 100, and/or other electrodes and sensors(not shown), and evaluates these inputs to detect an onset of cardiacarrhythmia. Preferably, block 82 employs techniques known in the art fordetermining arrhythmia, so that control unit 90 can treat or terminatethe arrhythmia by pacing or by performing cardioversion ordefibrillation. In a preferred embodiment, control unit 90 applies ashockless defibrillation technique, as described in U.S. ProvisionalPatent Application No. 60/136,092, entitled “Shockless defibrillation,”which is assigned to the assignee of the present patent application andis incorporated herein by reference.

As described hereinabove, the motion sensor signals typically providefeedback to enable the control unit to modify the electrical signalsapplied to the heart, in order to reduce the detected motion of thesegment. Additionally or alternatively, local sense electrodes 74, whichoptionally comprise some or all of electrodes 100, convey electricalsignals to control unit 90 to enable parameter search and tuning block84 to synchronize the electrical signals applied by electrodes 100 withthe natural electrical activity of the heart and with propagationcharacteristics of the applied electrical signals. Preferably, parametersearch and tuning block 84 assesses the output from local senseelectrodes 74 in conjunction with the motion sensor signals, so as todetermine appropriate parameters for the applied electrical signals,which both minimize motion of segment 24 and preserve the overallfunction of heart 20.

In a preferred embodiment of the present invention, some of electrodes100 apply rapid pulses to segment 24 which are generally similar in formand intensity to pulses commonly used to pace the heart. The pulses arebelieved to induce a reversible state of generally constant contractionof the segment, without causing fibrillation or other dangerousarrhythmic activity. In a preferred rapid pulse application mode,control unit 90 generates a regularly-spaced series of current pulses,injecting current through the electrodes into underlying cardiac tissue.In this mode, the pulses are preferably characterized by a frequencyabove 5 Hz, and are typically applied above 10 Hz. Pulses appliedbetween about 25 and 30 Hz have been found by the inventors to producegenerally desirable results. Other parameters typically characterizingthe pulses include a duty cycle between about 5 and 50%, a DC offsetbetween about −10 and +10 mA, and an amplitude between about −20 and +20mA. An amplitude of between about 1 and 5 mA is typically sufficient.These values are cited by way of example, however, and it will beunderstood that higher or lower frequencies and amplitudes may also beused, depending on the type and placement of the electrodes and on thespecific condition of the patient's heart. For example, a frequencyhigher than 100 Hz was tested on rabbits and found to yield suitableresults.

Alternatively or additionally, control unit 90 applies a fencing signalto some of electrodes 100 (or to other electrodes, not coupled to thestabilization element), generally in order to inhibit the generation andpropagation of an action potential from one region of the heart toanother. Fencing is typically used in these applications to block orreduce the normal propagation of signals and/or to reduce thecontractility of affected muscle tissue. Alternatively or additionally,the fencing signal generally reduces the contraction strength of themuscle stimulated thereby.

In a preferred embodiment, the electrical signals comprise first andsecond electrical signals, which are respectively applied to first andsecond sets of electrodes 100. Preferably, the first and second signalshave respective first and second frequencies associated therewith, whichgenerate electric fields in the heart at the respective frequencies.Typically, the first and second signals have frequencies between about500 and 20,000 Hz, and the difference between the first and secondfrequencies is between about 4 and 25 Hz. It is believed that thesegment's motion is reduced responsive to a beat frequency generated byinterference of the first and second signals. Suitable methods andapparatus for applying the first and second signals, mutatis mutandis,are described in the above-mentioned US patent application, entitled“High-frequency induction of cardioplegia.”

Alternatively or additionally, the electrical signals applied bystabilization element to segment 24 comprise an amplitude-modulated (AM)signal, applied by control unit 90 to one or more of electrodes 100. TheAM signal comprises (a) a high-frequency component, usually over 500 Hz,which generally passes through cardiac tissue, substantially withoutaffecting cardiac function, and (b) a low-frequency component, generatedby modulation of the amplitude of the high-frequency component. Thelow-frequency, similar to the beat frequency described above, ispreferably between about 4 and 25 Hz. Preferably, the AM signal isapplied in a manner generally similar to that described in theapplication “High-frequency induction of cardioplegia.”

In general, each one of electrodes 100 conveys a particular waveform toheart 20, differing in certain aspects from the waveforms applied by theother electrodes. The particular waveform to be applied is determined bycontrol unit 90, preferably under the control of a human operator.Aspects of the waveforms which are set by the control unit, and maydiffer from electrode to electrode, typically include parameters such astime shifts between application of waveforms at different electrodes,waveform shapes, amplitudes, DC offsets, durations, frequencies, dutycycles, etc. For example, although the waveforms applied to theelectrodes typically comprise a series of monophasic square wave pulses,other waveforms, such as a sinusoid, a series of uniphasic and/orbiphasic square waves, or substantially any other shape known in the artof applying electric signals to tissue, could be used in the frameworkof the present invention. Additionally, in some operational modes, thevoltage applied by some or all of electrodes 100 is controlled, ratherthan the current, as described hereinabove.

Generally, the shape, magnitude, and timing of the waveforms areoptimized for each patient, using suitable optimization algorithms, asare known in the art, in order to attain a desired level ofstabilization of segment 24. Typically, the optimization is performedcontinually, both during the calibration period and during regularoperation. However, during a surgical procedure, the operationalparameters are typically changed more gradually, so as not to interruptthe surgeon's actions.

Preferably, application of the electrical signals in accordance with thepresent invention increases the stability of segment 24 within a veryshort period (e.g., several seconds), such that the surgeon preferablyapplies mechanical force via stabilization element 25 to the segmentwhen the segment is already at least partially stabilized. In thismanner, a lower amount of mechanical force is typically applied to thesegment than would be applied using prior art methods. It is believedthat the lower force is likely to induce substantially less trauma tothe heart compared with results obtained using prior art methods forcardiac mechanical stabilization. The inventors have found that theheart typically returns to normal function within about 2 seconds ofremoval of the electrical signals. A short waiting time, typically about15 seconds, is preferably followed by recalibration before signals areapplied again. Although the initial calibration period can take severalminutes in order to determine appropriate signals to be applied byelectrodes 100, recalibration typically requires less time. The methodof this embodiment of the present invention has been found to begenerally spontaneously reversible, typically without requiringcardioversion or defibrillation. (Cardioversion and defibrillationcapabilities are nevertheless typically provided to enhance safety.)

Control unit 90 preferably comprises a flow control block 88, typicallyincluding valves and mechanical switches. Block 88 allows the controlunit to regulate the flow of liquid and gas from source 92 to thesurface of heart 20, as described hereinabove, and, additionally, tocontrol the timing and/or strength of the vacuum applied through ports39. Depending on the nature of the surgical procedure, the operation offlow control block 88 may be regulated directly by operator controls 71and/or by parameter search and tuning block 84, responsive to the inputsthereto. In another preferred embodiment (not shown), the strength ofthe vacuum and/or the flow of liquid and gas to the heart is regulatedindependent of the control unit.

Although preferred embodiments are described hereinabove with referenceto reducing motion of the segment of the heart in order to enablesurgery on the segment, it will be understood that the present inventionmay be used for other purposes, such as to enhance a physician's abilityto perform diagnostic tests on the segment. Furthermore, the principlesof the present invention are applicable not only to the heart, but alsoto controlling local motion in segments of other types of tissue, suchas smooth muscle (e.g., the intestines) and skeletal muscle.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and subcombinations of the various featuresdescribed hereinabove and in the articles, patents and patentapplications incorporated herein by reference, as well as variations andmodifications thereof that are not in the prior art, which would occurto persons skilled in the art upon reading the foregoing description.

1. Apparatus for use during performance of a surgical procedure on asegment of a beating heart, comprising a surgical tool, which comprises:a mechanical stabilization element, a surface of which is adapted to beapplied, during the surgical procedure, to the segment of the heart toreduce motion of the segment; and one or more electrodes, fixed to thesurface of the stabilization element, so as to contact the segment whenthe stabilization element is applied to the segment.